MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the barrier. The breakdown is visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focused ion beam out of the junctions characterized by transport investigations. A direct comparison of transport behavior and structure of the intact and broken junctions is obtained. The MgO barrier shows many microscopic pinholes after breakdown. This can be explained within a model assuming a relationship between the current density at the breakdown and the rate of pinhole formation. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3001934]